They tried making Thor ultra serious, and it was terrible.
The first Thor had the degree of fish out of water, where seeing him learn about earth culture was a spectacle. The Dark World was just not fun. Ragnarok decided to take Thor down the route of a comedy adventure (much like GotG) and it worked, big time.
The entire cast of Ragnarok was fantasic. This was the most fun we've had with Thor, Loki, and especially Hulk. Banner was great. Valkyrie was a new take that I think really worked. Korg was hilarious. Hela was a good villain, but once you sprinkled in Surtur, Grandmaster, small parts by Executioner and Fenris Wolf, with a touch of Loki's betrayal, it was a pretty solid ride.